Author Topic: Transistor basics - falling curve on emitter side?  (Read 961 times)

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Offline bitmanTopic starter

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Transistor basics - falling curve on emitter side?
« on: January 28, 2018, 12:07:35 am »
Hello, I have another newbie question. This time about transistors (a basic 2n2222). In the image below you can see the result of a simple curcuit using a toggling jk flipflop where the two output lines are each driving a 2n2222 transistor. I'm experimenting a bit to get better fanout as these clocks is driving 8-10 different ICs (7400 series).

What I don't understand is the 4ms curve when the transistor turns off. Note - I'm using the NPNs where the load is on the emitter side - not the collector side. And that's my question. When used on the collector side I get fairly flat pulses - but this looks like a capacitor is in play and there's noe attached to the flip-flop. The input signal (the yellow one) is flat, and that's where the only capacitor is found.

5v
 |
 2n2222  -- in signal from flip-flop
 |
 (load - in this case just an LED with a 330ohm resistor)
 |
gnd

Sorry, crude drawing. I wanted to measure 0->5v so I thought putting the load on the emitter side would allow me to plot that out, but this is what I get. I understand that there is some resistance in the transistor - but obviously it looks like there's some kind of inductance too?   This also seems to be within tollerances on the switching characteristics on the basic 2n2222 (25ns raise time) - I'm using a whopping 2.0 ms way above that.  And as I stated if it is the transistor I would expect to see the same curve on the collector side? Since I don't I doubt I need to keep looking at the transistor, but at something else.

Basic question: What's causing this? Outside of putting the load "on the right side" is there anything that can be done to change this?
 

Offline Audioguru

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Re: Transistor basics - falling curve on emitter side?
« Reply #1 on: January 28, 2018, 12:34:02 am »
You said the emitter load is an LED. But it is a load only while it is conducting:
 
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Offline bitmanTopic starter

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Re: Transistor basics - falling curve on emitter side?
« Reply #2 on: January 28, 2018, 12:38:37 am »
DUH - missing the obviously it looks like.  That must be it - as I have to eat my words that this circuit on the collector side behaves differently.  This is a single transistor, where I measure the base signal and the collector signal - and note the curve on the collector which again for test purposes is driving a LED. A cheap LED - so this really explains a lot. Thank you @Audioguru!
 

Offline bitmanTopic starter

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Re: Transistor basics - falling curve on emitter side?
« Reply #3 on: January 28, 2018, 01:10:52 am »
@Audioguru
I'll end this very short thread by confirming that it was indeed the LED I was measuring - not the transistor. I replaced the load with a simple resistor and presto - it's perfectly square. Lesson learned. Thanks again.
 

Online David Hess

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Re: Transistor basics - falling curve on emitter side?
« Reply #4 on: January 28, 2018, 07:08:36 am »
Base-emitter junctions are really good diodes with low reverse leakage so what you saw is actually the 1 or 10 megohm input resistance of the oscilloscope probe pulling the output down.
 


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